Why was Go a harder game for an AI to master than Chess?
The branching factor is important, as it limits the effectiveness of search.

However, the branching factor in chess is already too high to effectively search without techniques that reduce the size of the search space. Even with millions of tests per second, a computer can only check a small fraction of the possible future games in order to find results in its favour. 

One key factor is heuristics - approximate measures of the value of each game state. A good heuristic can guide and improve search by orders of magnitude. There are some effective heuristics possible in chess, from weighted values of the pieces in play, scores for areas of the board that a position controls etc. 

Heuristics for Go are much harder to find. Here's a sample paper from a few years ago that makes an attempt, there are several similar ones available online. Although plenty of options have been tried, and many were partially successful, none managed to bring the quality of computer play up to the standard of best human players. 

One of the major achievements of AlphaGo was training a neural network that had good position evaluation - the "value network". The technology that made generating this approximate function of board game positions possible was deep learning, which has been developed very strongly since about 2010. 

It is still possible a more analytical heuristic approach could be found that challenges deep learning models driven by self-play reinforcement learning on more raw board data. However, in some regards the reverse has been shown, with AlphaZero taking the same learning technique into chess and demonstrating its effectiveness against "old school" tuned expert heuristics.